1. Field of Endeavor
The invention relates to a premix burner for a combustion chamber of a gas turbine, especially in a power plant, at least having a housing defining a mixer chamber, an oxidator feed device for feeding a gaseous oxidator into the mixer chamber, a gaseous fuel feed device for feeding a gaseous fuel into the mixer chamber, and also a liquid fuel feed device for feeding a liquid fuel into the mixer chamber.
2. Brief Description of the Related Art
A premix burner of the type referred to above is known from EP 0 433 790. The generic type burner has a housing which is built from a plurality of internesting shells and which encloses a mixer chamber. By the offset arrangement of the half-shells, slots are formed for tangential feeding of an oxidator, especially combustion air, into the mixer chamber. Due to the tangential combustion air inlet, a swirled flow is formed in the mixer chamber and becomes unstable at the burner outlet due to a cross sectional jump, and changes into an annular swirled flow with a backflow in the core. This backflow enables the stabilization of a flame front downstream of the burner outlet. Injectors for injecting a gaseous fuel into the combustion air are provided inside the inlet slots for the combustion air. This injection, in conjunction with the turbulent swirled flow inside the mixer chamber, leads to a good mixing through of the gaseous fuel with the combustion air. A good mixing through in such burners is one of the preconditions for low NOx emissions during combustion. Furthermore, the burner is equipped with a central lance for feed of a liquid fuel, which extends from the burner head into the mixer chamber. The lance on its free-standing axial end has an injection orifice through which the liquid fuel is injectable into the mixer chamber and also into the combustion space of a combustion chamber, which combustion space is arranged downstream of the mixer chamber.
In the generic type burner, injection of the liquid fuel into the mixer chamber is carried out parallel to the burner axis, and injection of the gaseous fuel into the combustion air is carried out parallel to its flow direction. As a result, the characteristics of injection with regard to penetration depth and mixing in of the fuel jets, and also the fuel distribution along the combustion air inlet slots and also along the burner axis, are specified. The arrangement of the outlet orifices establishes the mixing qualities of fuel and combustion air, and also fuel distribution at the burner outlet. However, these variables significantly influence the NOx emissions and the quenching limit of the burner, and also its stability with regard to combustion pulsations.
The partial load range is problematical during operation of premix burners, especially such burners in connection with gas turbine plants, since in this case only comparatively low quantities of fuel are added to the combustion air. With complete mixing of the fuel with all the combustion air, however, a mixture results which even in the low partial load range is no longer ignitable, or only forms a very unstable flame. This leads to unwanted combustion pulsations or to a possible quenching of the flame.
A possibility for reducing these disadvantageous effects is to feed all the required quantity of fuel via the central lance. The burner is then operated at very high air ratios as a diffusion burner. On the one hand, a very high flame stability, but, on the other hand, also very high NOx emissions, result from this.
A further development of the burner which is discussed above is the subject of EP 1 292 795, which discloses a burner which, even during changes of load or changes of fuel quality, can be stably operated with approximately constantly low emission values. This premix burner includes a housing, which has one or more shells, a mixer chamber, into which combustion air is injected via tangentially arranged slots and which changes into a swirled flow in the mixer chamber, and means for introducing fuel into the combustion air flow, wherein this means has a first group of fuel outlet orifices for a first fuel, which are basically oriented parallel to the burner axis, and at least one second group of fuel outlet orifices for a second fuel, which are basically oriented parallel to the burner axis, wherein the first and the second group are subjectable to fuel admission independently of each other, and the means is preferably arranged in the region of the combustion air inlet slots.
For further increasing of the flame stability, pilot fuel can be additionally introduced via a lance.
Since the burner can be exclusively operated with liquid fuel, the possibility arises of maintaining or repairing the gaseous fuel feed device without the operation of the burner or of the combustion chamber having to be completely interrupted for this purpose. This is advantageous for the efficiency of the gas turbine which is equipped with it. As already mentioned elsewhere, however, injection of liquid fuel into the mixer chamber of the burner or into the combustion space of the combustion chamber, as the case may be, customarily leads to appreciably increased flame temperatures, which, for example, is to be ascribed to inadequate atomization, mixing and evaporation of the liquid fuel before its ignition. Increased flame temperatures, however, are associated with a disproportionally increased production of NOx emissions and soot. This disadvantage can be minimized somewhat by water, or water vapor, being admixed with the liquid fuel, for example in a quantity ratio of 1:1, and, instead of liquid fuel, a fuel/water emulsion consequently being injected into the mixer chamber, which leads to a delay of the combustion reaction and to a lowering of the local flame temperatures. In this case, it is again disadvantageous that the feed of such a thinning medium increases the heat transfer in the turbine on the hot gas side, which is accompanied by a reduction of the service life of the turbine. Furthermore, there are sites for power plants in which water is too expensive to be used as a thinning medium. Furthermore, the comparatively short time in which the burner is actually operated with liquid fuel is taken into account, so during a service of the gaseous fuel feed device, or in pilot mode, the costs for preparation of the water, for example dimineralization plants have to be made available for this, are therefore too high.